Ceramic composites are gaining emphasis in diverse applications such as heat engine components, cutting tools and various wear resistant parts. The ceramic composites have typically improved fracture toughness and improved wear properties. Conventional ceramics are generally monolithic materials and have low fracture toughness. This makes these materials brittle and they are liable to crack under stressed conditions, and are not very useful for diverse demanding industrial applications. Monolithic ceramic materials such as silicon carbide, alumina, silicon nitride and mullite have low fracture toughness of between 2.5 to 4.5 (K.sub.IC :MPa.sqroot.m).
There has been extensive research underway to produce ceramic composites of higher fracture toughness, using matrix such as alumina, silicon nitride and silicon carbide, reinforced by materials such as silicon carbide particles which may be formed, for example, as whiskers or fibres.
Alumina as a matrix material with silicon carbide whisker reinforcement for ceramic composites have received strong attention. Fracture toughness and strength of alumina-SiC whisker composites (Al.sub.2 O.sub.3 -SiC) are much higher than monolithic alumina. The improved strength and fracture toughness are retained to high temperatures of around 1200.degree. C. The reports on SiC whisker reinforced alumina matrix composites are so encouraging that the product is already being commercially produced as cutting tools, wear dies and other applications. Cutting tool materials typically contains around 30% SiC whiskers and show much better resistance to wear and fracture in machining operations. It has even been reported by E. R. Biollman, P. K. Mehrotra et al. Am.Cer.Soc.Bull., 1988, 67, 1016, that the estimated savings in field tests of machining a superalloy is 73% with the Al.sub.2 O.sub.3 -SiC composite tools as compared to machining with just Al.sub.2 O.sub.3 tools.
Typically these composites (G. C. Wei, U.S. Pat. No. 4,543,345) are produced by mechanically mixing single crystal SiC whiskers with fine ceramic powders such as alumina (Al.sub.2 O.sub.3) such that the SiC whiskers are uniformly and homogeneously dispersed. This homogeneous dispersion is normally difficult to achieve through mechanical mixing processes. The mixture is then densified through techniques such as hot pressing at pressures in the range of 28 to 70 MPa and temperatures in the range of about 1600.degree. to 1950.degree. C. with pressing times varying about 0.75 to 2.5 hours.
There are a number of major problems with the above ceramic composite and process for making same. The silicon carbide whiskers are very expensive as they are made primarily through a VLS process. The silicon carbide whiskers are extremely carcinogenic and are very dangerous to handle. The dispersion of the SiC whiskers is difficult to achieve and elaborate processing techniques are necessary. With mechanical methods of mixing whiskers and ceramic matrix powders, there is the possibility of whiskers clustering together and whisker damage, and the extent of whisker loading is limited. For example in order to obtain a good dispersibility of whiskers and thus improve the strength of the composite, both ultrasonic dispersion techniques and finer particle, non-agglomerated matrix powder have to be used (P. F. Becher and G. C. Wei, Journal of the American Ceramic Society., 1084, 67,C267). Very elaborate processing techniques, involving flotation or sedimentation from dispersions of the components, were found to be effective in eliminating the potential flaw types (J. Homery, W. L. Vaughn and M. K. Ferber in the American Ceramic Society Bulletin, 1987, 67,333). With the new information that the SiC whiskers are very carcinogenic, all these complex processing techniques have become very unpleasant.